(1) Field of the Invention
This invention relates to a method for the surface-hardening treatment of a synthetic resin shaped article and an apparatus for use in carrying out said method.
(2) Description of the Prior Art
Synthetic resins excellent in molding processability and impact resistance are used in various fields. However, most synthetic resins have problems such that the surfaces of shaped articles of these synthetic resins are not hard, are readily marred, and if mars and scratches are formed on the surfaces, the commercial values of the shaped articles are reduced. These problems are serious particularly in optically transparent materials such as lenses and light-transmitting transparent materials such as lamp lenses. It has, therefore, eagerly been desired to impart a scratch resistance to the surfaces of shaped articles.
A method has been proposed and adopted in which a silicone type cured coated film is formed on the surface of a shaped article to improve the surface hardness. As means for forming a uniform coating of a cured film, U.S. Pat. Nos. 3,956,540 and 4,036,168 disclose a method and apparatus for hardening the surface of a shaped article by applying a fluoroethylene polysilicate polymer as the surface-hardening liquid onto the surface of the shaped article. The hardening solution used in this conventional technique is cured by application of heat, and the curing must be carried out in two stages, namely, after the pre-curing step, the surface of the shaped article of a tack-free state must be further treated in the subsequent curing step for imparting thereto the desired mar or abrasion resistance.
Research was made in view of obtaining a synthetic resin shaped article having enhanced surface hardness and abrasion resistance by a method wherein a hardening solution curable with ultraviolet rays is applied onto the surface of a synthetic resin shaped article and whereby a uniform coating can be formed and the coating can be cured in a single curing step. As a result, the following facts were understood.
When a hardening solution is applied onto the surface of a synthetic resin shaped article, spittings, cissings or pinholes are readily caused, and in order to obtain a treated article excellent in the surface smoothness, it is necessary to circulate and filter the hardening solution to be coated on the surface so as to sufficiently remove solids and foreign substances from the hardening solution. Moreover, it is also necessary to clean the surface of an article to be treated by a preliminary treatment.
Furthermore, it is necessary to maintain a dust-free clean state in the atmosphere throughout the surface-hardening treatment zone, and it often is necessary to control the temperature and humidity of the atmosphere. Otherwise, such defects as unevenness of the thickness, whitening and orange peel are caused on the coating film by irregular evaporation speed or condensation of water on the coating surface. Moreover, the smoothness or hardness of the cured film is influenced by the period of from the point of coating of the hardening solution to the point of curing by irradiation with ultraviolet rays.